The present invention relates generally to liquid natural gas vaporization and more particularly to the recovery of boil-off gas vapors from n liquefied natural gas (LNG).
Natural gas liquefaction by cryogenic cooling is practiced at remote natural gas rich locations to convert the natural gas to a transportable liquid for shipment to available markets. Before and after transport, the liquefied natural gas (LNG) is stored in a storage tank. Evaporation inside the tank causes the LNG to boil off natural gas vapors, which collect in the storage tank. The vapors generated are defined as boil-off vapors and are low-pressure natural gas. These boil-off vapors would eventually cause the tank pressure to rise above its design pressure if not appropriately addressed.
Various procedures exist in the art for dealing with these vapors including venting the vapors into the atmosphere, compressing the vapors to pipeline or process pressure and injecting them into the pipeline or process. These procedures, however, are not environmentally friendly and generally are not economically viable. To avoid these problems, boil-off vapors can be recondensed into the LNG that is being pumped to pipeline pressure (via booster pumps), which in turn is vaporized and injected into the pipeline. Not only is such procedure environmentally friendly and economically viable, but is also efficient because all of the natural gas, including its vapors, is utilized.
Natural gas recondenser systems in the industry typically require complicated internal components for the recondenser (such as pall rings, separator devices, deflector plates, internal piping, etc.). In addition, recondenser systems used in the industry typically require a liquid level to be maintained in the recondenser vessel, which also requires high-pressure make-up gas (pipeline gas or nitrogen) to be injected into the recondenser to force a liquid level in the recondenser. Some of these systems also contain separate heat exchangers, as well anda complex controls, equipment, and valving.
It is desired therefore to have a natural gas recondenser system that recondenses boil-off vapors from a LNG tank via a simplified system with less equipment and less valving. It is also desired to halve a natural gas recondenser system that does not require a liquid level to be maintained in the vapor/liquid separator drum (recondenser) and hence does not require any high-pressure make-up gas. In addition, it is desired to have an efficient natural gas recondenser system that recycles excess vapor back to the LNG tank to have another chance to be recondensed.
One embodiment of the present invention is a process for re-liquefying boil-off gas produced by liquid natural gas. The process contains the steps of (a) containing liquid natural gas in a collector (LNG storage tank), wherein the liquid natural gas produces boil-off gas, (b) compressing and cooling the boil-off gas, to form a higher pressure vapor stream, (c) boosting the pressure of the liquid natural gas to form pressurized liquid natural gas, (d) combining this cooled vapor stream with the pressurized natural gas in a mixing device, to form a substantially liquid gas stream, (e) separating in a separating device any vapor, if vapor exists, from the substantially liquid gas stream to form a vaporless liquid gas stream, (f) returning any vapor from the separating device to the LNG storage tank, and (g) repeating steps (a) to (f).
Another embodiment of the present invention is an apparatus for re-liquefying boil-off gas produced by liquid natural gas. The apparatus is made up of a collector (LNG storage tank), wherein the LNG storage tank contains liquid natural gas, at least one first pump, wherein the first pump is in or adjacent to the LNG storage tank, a compressor, wherein the compressor communicates with the vapor space in the LNG storage tank by a first conduit, at least one mixing device, wherein the mixing device communicates with the compressor by a second conduit, and wherein the mixing device communicates with the first pump by a third conduit, at least one separating device, wherein the separating device communicates with the mixing device by a fourth conduit,and wherein the separating device also communicates with the vapor space in the LNG storage tank by a fifth conduit.